🧪 Add missing tests for contactsById dictionary

[Sparkier]

• 🎯 What: The testing gap for the contactsById dictionary (missing tests entirely) was addressed.
• 📊 Coverage: The tests now explicitly cover that contactsById maps every ID correctly to the matching object in the contacts array, and verifying it has exactly the same length.
• ✨ Result: Improved test coverage and reliability by explicitly testing the dictionary alongside the array.

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PR created automatically by Jules for task 11276687523353287574 started by @Sparkier

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